The California-based Virgin Galactic are back on track, however, as they gear up to launch SpaceShipTwo's new, safer model, VSS Unity.

If it passes extensive testing, it will offer the world's first commercial spaceflight programme.

In a statement last month, Virgin Galactic said:

"When we are confident we can safely carry our customers to space, we will start doing so.

"We feel incredibly honoured that our earliest paying customers already number more than the total number of humans who have ever been to space.

"Our first spaceflight with paying customers; our first flight full of research experiments; our first flight with a full complement of eight... the dozens of times we will fly the first ever astronaut from a given nation. Each of these will be exciting milestones in the history of space exploration".